Various industries, such as oil exploration and mining, require techniques for investigating underground conditions without the expensive proposition of random drilling or excavation. Various techniques have been developed for exploring subsurface conditions with a minimum of effort and expense. One such technique is borehole seismic logging. Borehole seismic logging techniques involve lowering an energy source within a deep borehole, which is a hole several inches wide and hundreds of thousands of feet deep drilled into the earth's surface. The energy source is designed to generate seismic waves in the borehole. These seismic waves are detected by one or more receivers which are placed in other boreholes, on the earth's surface, or in the same borehole where the energy source is located. The seismic waveforms that are picked up by the receiver or receivers, when compared to the known output of the energy source and processed with seismic data processing methods, can give the trained user the ability to map the underground formation to locate oil and gas.
The applications of these energy sources and receivers require that the energy source and receiver be capable of selective generation and detection of specific frequencies, or bands of frequencies, within the broad acoustical spectrum. Increasing the distance between the energy source and the receiver, while still maintaining the ability to detect the seismic waves from the energy source, makes borehole seismic logging techniques less expensive. While higher frequencies (2000 kHz and higher) are used in SONAR and similar acoustic techniques under water, experimentation has shown that the earth attenuates seismic waves above about 2 kHz, rendering frequencies above that range less useful in borehole seismic logging when the energy source and the receiver are relatively far apart (more than 700 feet).
Cylindrical piezoelectric crystals have been used as transducers to create seismic electric signal of a known characteristic, which is amplified by an associated power amplifier, to drive piezoceramic material located within the borehole. The electric signal excites the piezoelectric material causing the cylinder to change diameter and emit seismic waves in the hoop (or barrel) mode. Lengthening the ceramic cylinder creates seismic waves of lower frequencies. These arrangements require multiple crystals to general seismic waves of desired relative amplitude over a broad frequency range. Prior art energy sources lacked significant energy output below 700 to 1,000 Hz due to the "piezoelectric effect" in which power output from a piezoelectric material decreases as frequency is lowered.
Thus, a number of apparatus have been employed in the past in this field. For example, U.S. Pat. No. 4,319,345, entitled "Acoustic Well-Logging Transmitting and Receiving Transducers" and issued on Mar. 9, 1982 to Dennis, discloses an acoustic well-logging transmitting transducer employing a transmitter portion having stacked piezoceramic rings and a resonating metallic plate. The dimensions of the plate determine the frequency of resonation. A conical acoustic reflector causes reflections to impinge omni-directionally on the wall of the borehole at an angle to enhance shear wave component propagation.
Additionally, U.S. Pat. No. 4,400,805, entitled "Acoustically Filtered Transducer" and issued on Aug. 23, 1983 to Nadler, discloses an acoustic transducer including a pressure tight vessel having a window therein. The window is transparent to acoustic wave energy. A gas is disposed in the vessel, and an electromechanical transducer is located within the vessel at an antinode for a resonant wave of the gas. A port communicates with the interior of the vessel, with a pressure control coupled to the port and to a source of the gas for controlling the pressure of the fluid within the vessel.
Another apparatus, disclosed in U.S. Pat. No. 4,415,998, entitled "Segmented Acoustic Transmitter for Broad Frequency Investigation of a Borehole" and issued on Nov. 15, 1983 to Blizard, discloses cylindrical piezoelectric crystals used as a transducer to create a seismic electric signal. The signal is amplified by an associated power amplifier to drive piezoceramic material located within the borehole. The electric signal excites the piezoelectric material causing the cylinder to change diameter and emit seismic waves in the hoop (or barrel) mode. Lengthening the ceramic cylinder creates seismic waves of lower frequencies. These arrangements require multiple crystals to general seismic waves of desired relative amplitude over a broad frequency range.
U.S. Pat. No. 4,493,062, entitled "Resonant Frequency Modification of Piezoelectric Transducers" and issued on Jan. 8, 1985 to Mallett, discloses a transducer modification circuit for use with an acoustic transmitter in an acoustic well logging tool. The circuit modifies the diameter resonant frequency of a piezoelectric transducer, thereby expanding the frequency range downward for the transmitter transducer by moving the diameter resonant frequency.
In U.S. Pat. No. 4,890,687, entitled "Borehole Acoustic Transmitter" and issued on Jan. 2, 1990 to Medlin, et al., an acoustic transmitter for use in a borehole logging tool that employs multiple Helmholtz resonators stacked such that apertures in opposite side of each of the resonators are in linear alignment is disclosed. Thus, increased acoustic energy output over a broad range of low frequency seismic frequencies is provided.
Yet another apparatus known in the field is disclosed in U.S. Pat. No. 5,081,391, entitled "Piezoelectric Cylindrical Transducer for Producing or Detecting Asymmetrical Vibrations" and issued on Jan. 14, 1992 to Owen. Here, a transducer constructed of a cylindrical shell and at least one pair of piezoelectric sections is disclosed. Each member of the pair of piezoelectric sections is rigidly bonded to the wall of the cylinder, in a position diametrically opposed from the other member of the pair. Operation as an acoustic source transducer is accomplished by electrically energizing each piezoelectric section to cause the piezoelectric sections to elongate and contract, respectively out of phase with one another, along the cylinder length. This causes flexural bending of the cylinder, with the bending being asymmetrical with respect to the cylinder axis. The transducer thus approximates an acoustic dipole radiator. Use of more than one pair of piezoelectric sections permits use of the transducer to approximate higher order polarized acoustic radiators.
In U.S. Pat. No. 5,115,880, entitled "Piezoelectric Seismic Vibrator with Hydraulic Amplifier" and issued on May 26, 1992 to Sallas, et al., a piezoelectric seismic vibrator using a hydraulic system to amplify the longitudinal displacement which results from the application of a voltage to a stack of piezoelectric elements is shown. The stack of piezoelectric elements is mounted to bear upon a power piston which, in turn, acts upon a substantially incompressible fluid, such as mercury. A drive piston has a cross-sectional area which is smaller than the area of the power piston. The pressure within the fluid system acts to amplify the longitudinal displacement of the piezoelectric elements. The drive piston movement is coupled mechanically or fluidically to the earth to generate seismic waves. Each stack of piezoelectric elements is disposed within a fluid filled chamber which is pressurized by communicating with the borehole fluids.
U.S. Pat. No. 5,184,332, entitled "Multiport Underwater Sound Transducer" and issued on Feb. 2, 1993 to Butler, discloses a multiport underwater sound transducer including a hollow resilient housing enclosing a volume with at least two ported resonant chambers and a transduction driver contained within the volume. Opposite sides of the driver drive the two chambers. The two ports are set to resonate at slightly different frequencies. The transducer produces an additive output at frequencies between the two slightly different frequencies due to phase reversals of oppositely phased sound waves.
A transducer to provide acoustic transmission in a borehole is shown in U.S. Pat. No. 5,283,768, entitled "Borehole Liquid Acoustic Wave Transducer" and issued on Feb. 1, 1994 to Rorden. The transducer has magnetic circuit gaps and electrical windings that provide the power necessary for acoustic operation in a borehole.
U.S. Pat. No. 5,331,604, entitled "Methods and Apparatus for Discrete-Frequency Tube-Wave Logging of Boreholes" and issued on Jul. 19, 1994 to Chang, et al., discloses a source transducer emitting acoustic waves of at least one discrete frequency which includes propagation of tube waves in a borehole. Acoustic energy of the tube waves is received at multiple receiver locations. The multiple receiver locations are spaced apart from each other and from the source transducer location. At each receiver location, the complex pressure response of a receiver to the received acoustic energy of the tube waves is detected relative to the phase reference. The detected complex pressure response is processed to determine phase velocity of the tube waves in the borehole as a function of frequency, to determine attenuation of the tube waves in the borehole as a function of frequency and/or to determine attenuation and amplitude as functions of frequency. Tube wave reflection coefficients near fractures and bed boundaries are thus calculable.
Finally, U.S. Pat. No. 5,360,951, entitled "Earth Reaction Seismic Source" and issued on Nov. 1, 1994 to Turpening, discloses a source for emission of seismic energy waves into subsurface earth formations. Electrically energized signal generators are mounted between two plate members. One of the plate members is anchored to the earth surface and the other rests on and is coupled with the earth surface. The source is moveable between shotpoint locations and provides controllable seismic signals of improved bandwidth and quality.
For the foregoing reasons, there has been defined a long felt and unsolved need for a resonant cavity piezoceramic borehole energy source that is easily installed, inexpensive to manufacture and compact enough to approximate a point source of acoustic energy.
In contrast to the foregoing, the present invention constitutes a dual mode multiple-element resonant cavity piezoceramic borehole energy source that seeks to overcome the problems discussed above, while at the same time providing a simple, easily constructed apparatus that is readily adapted to a variety of applications.